1. Field of the Invention
This invention relates to a disk apparatus for magnetically recording information on a disk, and more particularly, to the servo control of synchronous rotation of recording disks utilizing a rotational arm for actuating read/write heads, and, further, to a system in which index marks for indicating disk rotation are magnetically written on each cylinder deviated from a radial line of the disk so that the index marks on neighboring cylinders are aligned with each other.
2. Description of the Related Art
As shown in FIG. 1, in a magnetically recorded disk system, a plurality of recording disks 1 rotate simultaneously on a single spindle 2a. Magnetic heads 3 for reading/writing data on the disks are moved simultaneously by a head actuator 4 to select and maintain the heads on a cylinder. The head actuator 4 is driven by a servo controller 5. To seek a particular cylinder, as well as to detect rotation of the disks, a servo control surface 1a is provided as one of the surfaces of the disks 1, and a head 3a, for reading information from the servo control surface 1a, is also provided. The servo control head 3a reads out servo control information SVS, including index marks and guard band patterns, magnetically written on each track on the servo control surface 1a. The index marks are written with a servo control track writer (not shown in FIG. 1) between a first and last sector of each cylinder. The index marks are extracted from the servo control information SVS by an index pulse generator 50 and a signal is generated indicating the occurrence of a single rotation of the disks. The signal is fed back via a motor control circuit 6 to spindle motor 2, which rotates the disks to control the rotation of the disks. The index mark typically consists of sequential bit positions. Each bit of the servo control information SVS has a magnetized pattern. A group of these bits make up a servo control byte, a typical example of which is shown in FIG. 2(a), where W1 and W2 denote adjacent cylinders, and N (North) and S (South) denote the magnetic polarity. Si.sub.1 and Si.sub.2 are index bits, and Ss is a sync or reference bit. The bits from Si.sub.1 to Si.sub.2 make up a single servo control byte. The sync bit Ss is written in each servo control byte, and control generates a clock signal when read by servo control head 3a. The index bits Si.sub.1 and Si.sub.2 are used to code and define an index mark or a guard band. The servo control head 3a travels along the border line between two adjacent cylinders, overlapping each cylinder. The area R covered by servo control head 3a on adjacent cylinders W1 and W2 of FIG. 2a is indicated by dashed lines. Other heads 3 for reading/writing stored data travel along a single track corresponding to W1 or W2. A waveform detected by the servo control head 3a is shown in FIG. 2(b). As is well known, the detected signals "Su", produced from the bit position "u" of the cylinder W1 and "Sw" produced from the bit position "w" of the cylinder W2, indicate the location of the servo control head 3a with respect to the adjacent cylinders W1 and W2. Concurrently, a bit position mark written on both adjacent cylinders, such as the bit position mark "v", produces a signal Sv" having a large amplitude. The index marks indicating rotation of the disk produce these pulses. Therefore, if the magnetic pattern of the written bits (lines 100-107 in FIG. 3(a)) on adjacent cylinders are not aligned with each other as shown in FIG. 3(a), peaks of the signals detected therefrom are distorted as shown in FIG. 3(b). These distorted peaks lower the accuracy of disk rotation detection and tracking accuracy.
Recently, an apparatus for magnetic recording has been developed for use in a computer system in which several disk apparatus are connected in parallel. Rotation of the disks in each disk apparatus must be synchronized with each other, i.e., locked with an external synchronization pulse, as described in an unexamined Japanese Pat. No. Sho 59-218671 by T. Negoro. In this system, the index mark must generate an accurate timing signal indicative of the disk rotation.
However, in a compact disk system, a rotational arm is generally used for moving the heads across a cylinder. In this case, the magnetized pattern of the written bits 100-107, which are parallel to the core gap of the head, are not always perpendicular to the movement of the cylinder. Therefore, the read/write head on a rotational actuator is generally provided with an azimuth (which is an angle between the core gap of the head and the disk's radial line on which the head is located) as well as an offset angle (which is an angle of deviation of the core gap of the head from the tangential line of the rotational movement of the head). The size of the azimuth and the offset angle are chosen so that the azimuth is zero on the most inner cylinder where the read signal level is the smallest. Then, the waveforms are distorted as described above. Accordingly, the azimuth varies depending on the cylinder on which the head is located. The disk is provided with a mechanical index, such as a single optical mark, which is detected by an optical detector. If the index marks are written at a constant phase difference with respect to a reference position, (the mechanical index denoted by MIX of FIG. 4), the magnetized marks on each cylinder are not aligned with the marks on adjacent cylinders, as shown by IS1 through ISn in FIG. 4. (FIG. 3(a) also shows index marks which are not aligned with each other). Therefore, the locations of the index marks are shifted on each cylinder so that the magnetized marks of adjacent cylinders are partially aligned with each other as shown by IS2' through ISn' in FIG. 4, and as disclosed by M. Sasaki in an unexamined Japanese Pat. No. Sho 59-48872. By this method, though there is no discontinuity of the magnetized marks, and thus, the detected signal is not distorted, the index pulses I.sub.2 through I.sub.n detected from the written index marks IS2' through ISn' are delayed with respect to the timing of the mechanical index pulse MIX as shown in FIG. 5. In FIG. 5, the pulses written with dashed lines represent the timing of the undelayed index signals. For the synchronous control of the plural disk apparatus, the servo control is carried out so a phase difference between an external rotational synchronous pulse RSP and each of the index pulses I.sub.1 through I.sub.n becomes zero. Therefore, the above-described delay of the index pulses causes a large phase change in the servo control circuit when the heads jump across cylinders to seek a new cylinder. As an example, the amount of the phase difference on the cylinders for a disk of 10.5 inches in diameter, having 1024 cylinders thereon, can be great. The phase difference between the index marks on adjacent cylinders is approximately 40 nanoseconds (ns). Therefore, when the head moves from the most inner (or outer) cylinder to the most outer (or inner) cylinder during a search, it takes approximately 40 microseconds (.mu.s). This time, even though reduced when searching from a point between the inner and outer cylinders, is approximately 0.24% of a rotating cycle time of 3600 rpm. This is more than enough to cause an error when attempting to search a cylinder within a specified period, such as a single sector period. This is because the response characteristics of the servo loop of the motor control system may overshoot the cylinder being searched. This can cause the synchronizing servo to be disturbed to the extent that the servo loses control of the heads. Additionally, the synchronous parallel reading of the plural disk apparatus requires the above-mentioned delay time.